Gas turbine engines contain compressor blades which can experience centrifugal forces exceeding 15,000 g's because of the high rotational speeds at which they operate. It is clearly desirable to reduce the mass of such blades, and this reduction is commonly done by drilling holes 3 in FIG. 1 in the root region 6 of a blade 7. Such holes are termed lightening holes, because they lighten the weight of the blade.
The interior surface of such holes 3 must be smooth. A nonsmooth surface, such as one having a scratch produced by an imperfect drill bit, can cause a stress riser to occur at the location of the scratch. Such a stress riser is not desirable in blades which are subjected to the large centrifugal forces discussed above. It is desirable to have an apparatus for examining the interior of the hole 3 for detecting scratches or other defects.